Many marine cnidarians, and especially the Portuguese Man-of-War (Physalia physalis), inflict excruciatingly painful and often dangerous nematocyst stings to thousands of American bathers each year. Especially dangerous and occasionally fatal are those encounters with humans who have previously been stung and become hypersensitive to nematocyst venom. Little is known about the nature and mechanism of action of Physalia nematocyst venom; consequently, treatment for stinging victims is entirely supportive as no antidote or antivenom is currently available. The only nematocyst venom that has been obtained in pure form and extensively studied at the cellular and molecular level is that obtained from the sea anemone, Aiptasia pallida. Aiptasia nematocyst venom is composed of a mixture of proteins which have been physically separated. The venom contains only two types of toxins: a cytolytic enzyme (phospholipase A) and a very potent and specific neurotoxin, which also can act synergistically with the phospholipase A as a direct lytic factor (DLF). The activities of these two toxins and their biochemical mechanisms of action have been, and currently are being, studied in detail. There is much evidence to indicate that Physalia nematocyst venom is very similar to Aiptasia venom. We have developed methods for obtaining large quantities of highly purified Physalia nematocysts. The physical purification of the toxic components of Physalia venom will involve combinations of standard biochemical fractionation procedures. Once purified the individual Physalia toxins will be studied on standardized hemolysis, phospholipase A and mouse bioassays. Antibodies (Ab) will be produced in rabbits to the attenuated toxins and attenuated whole venom. Labelled Abs will be employed as agents to study the in vivo and in vitro mechanisms of action of the toxins. Abs will also be used to neutralize the venom's toxicity and confer passive immunity to rabbits. The attenuated Physalia venom or "toxoid" will be used to develop the optimal conditions for producing active immunity in rabbits and for desensitizing guinea pigs experimentally hypersensitized to Physalia venom.